1. Field of the Invention
The present invention relates to a process for the preparation of terephthalic acid by the liquid phase oxidation of paraxylene. More particularly, the present invention relates to the production of terephthalic acid at high productivity which acid is of sufficient purity so that it can be directly polymerized with ethylene glycol to form polyester without complicated and/or expensive purification.
2. Description of the Prior Art
There is a recognized economic benefit in continuously producing terephthalic acid of sufficient purity so that the acid can be directly polymerized with ethylene glycol after relatively simple methods of separation and purification. Various modifications to the known procedures for the oxidation of paraxylene to produce such fiber-grade terephthalic acid have been proposed. These include changing oxidation conditions, catalysts levels and composition, and recovery of terephthalic acid. However, the modifications are frequently in conflict. British Pat. No. 1,249,198 published on Oct. 6, 1971 describes a process which allegedly produces terephthalic acid of high purity and reduced color by conducting the oxidation at a maximum temperature of 200.degree. C. using specified amounts of manganese, bromine and a specified atomic ratio of cobalt to manganese. British Pat. No. 983,677 published on Feb. 17, 1965 describes the production of terephthalic acid of enhanced purity by two stage oxidation with the second stage using gaseous molecular oxygen at temperatures higher than those employed in the first stage and usually above 200.degree. C. and preferably 230.degree. to 280.degree. C. The use of air diluted with an inert gas (5 to 17 volume percent oxygen) is alleged to produce high purity terephthalic acid in unexamined Japanese Patent Application No. 50-30839 published on Mar. 27, 1975. A number of catalysts have also been proposed. U.S. Pat. No. 3,846,487 issued on Nov. 5, 1974 alleges beneficial results by using cobalt bromide and free bromine (Br.sub.2) exclusively as the source of bromide whereas bromoform as the sole source of bromine is alleged to have a beneficial effect in unexamined Japanese Patent Application 51-43734 published on Apr. 14, 1976. Although alkali metal ions, e.g., sodium are taught to be beneficial in U.S. Pat. No. 2,959,613 issued on November 8, 1960, the presence of such ions are taught to be detrimental to the production of relatively pure terephthalic acid in U.S. Pat. No. 3,970,696 issued on July 20, 1976. The use of hydrogen bromide as a source of bromine along with an alkali metal is taught to produce high purity terephthalic acid in Japanese Pat. Application 51-70741 published on June 18, 1976. Mole ratios of cobalt to manganese of at least about 4.7 up to 91 are taught to be beneficial for producing terephthalic acid of high purity in unexamined Japanese Patent Application 45-36732 published on Nov. 21, 1970.
German Pat. No. 2,534,161 opened for inspection on Feb. 12, 1976 teaches that maintenance of 4-carboxybenzaldehyde in the mother liquor of the terephthalic acid slurry in the oxidation vessel at less than 800 parts per million provides a high quality terephthalic acid.
Certain mechanical arrangements are also taught to be beneficial to the production of highly pure terephthalic acid. U.S. Pat. No. 3,839,435 issued on Oct. 1, 1974 discloses introducing paraxylene in at least three inlets distributed at differing heights in the vertical plane of the reaction vessel. Unexamined Japanese Patent Application 47-14092 published on Apr. 27, 1972 teaches introducing paraxylene into the reactor by passing it through a hollow stirring shaft with a multiple number of hollow stirring blades so that the paraxylene is fed through the openings of the rotating stirring blades and thereby is more thoroughly dispersed in the reaction medium. Advantages are alleged by adding raw paraxylene to the condensate reflux of the reactor in unexamined Japanese Patent Application 49-20192 published on May 23, 1974.
Post treatment of the oxidate to reduce impurities is also taught to be beneficial. U.S. Pat. No. 3,859,344 issued on Jan. 7, 1975 discloses treating the oxidate with a molecular oxygen containing gas for a short period while the oxidate is intermittently withdrawn from the oxidation reactor. The use of an oxygen containing gas having an oxygen level less than that of air is disclosed to be beneficial in the post oxidative treatment of the oxidate as disclosed in unexamined Japanese Pat. Application 51-39642 published on Apr. 2, 1976. U.S. Pat. No. 3,507,913 issued on Apr. 21, 1970 discloses a technique for recovery of the reaction product from the oxidation by adding one-tenth to five times the volume of the reaction product of acetic acid before the temperature of the product decreases below 180.degree. C. and before the amount of precipated terephthalic acid exceeds 20% by weight based upon the weight of the liquor. The maintenance of a specific temperature during the recovery of terephthalic acid to enhance its purity is disclosed in U.S. Pat. No. 3,996,271 issued on Dec. 7, 1976.